When light passes the lens of a digital camera and is captured by the CCD it is converted into an electrical charge.
There is a limit to how much charge each pixel or photosite can store. If there is too much charge for one photosite it will overflow to its neighbouring pixel causing an effect which is called blooming or streaking. Manufacturers try to eliminate this effect by “anti-blooming gates” which can be compared to vertical drainage channels running beside each row of photosites. These allow the overflowing charge to flow away without affecting surrounding pixels.
Though these anti-blooming gates are fairly successful at avoiding the problem, very extreme exposure situations can still lead to blooming. Especially where a very bright edge is next to a very dark edge, as seen with leaves or branches of a tree shot against a bright sky. It will be visible as a white halo or vertical streak which extends for several pixels. The effects of blooming often make chromatic aberrations more visible. These are the purple lines along dark edges in an image, caused by the effect of blue light bending more than red light.